Dr. Tyndall on Molecular Influences. 133 



We further find that the lateral conductivity bears no defi- 

 nite ratio to the longitudinal conductivity. Indeed the tendency 

 appears to be that those woods which possess the lowest power 

 of transmission, along the fibre, possess the highest power across 

 it. But here the exceptions are so numerous that we have no 

 warranty for a general conclusion. 



But the most remarkable result of the experiments remains 

 yet to be stated. If we cast our eyes along the second and third 

 columns of the tabular summary, we shall find that in every 

 instance the velocity of propagation is greatest in a direction 

 perpendicular to the ligneous layers. This result is, of course, 

 wholly independent of the specific heat of the wood, inasmuch 

 as it is two directions through the self-same cube which are here 

 compared with each other. The law of molecular action, as 

 regards the transmission of heat through wood, may therefore 

 be expressed as follows : — 



At all the points not situate in the centre of the tree, wood pos- 

 sesses three unequal axes of calorific conduction, which are at right 

 angles to each other. The first, and principal axis, is parallel to 

 the fibre of the wood ; the second, and intermediate axis, is perpen- 

 dicular to the fibre and to the ligneous layers ; while the third and 

 least axis is perpendicular to the fibre and parallel to the layers. 



The researches of Savart on the sonorous vibrations of wood 

 naturally suggest themselves here; for, doubtless, the same 

 molecular structure which imparts to this substance the peculiar 

 elastic properties discovered by Savart, must be regarded as the 

 cause of the differential action established above. Savart* took 

 bars of equal size, and in different directions, from a mass of 

 wood ; determining their resistance to flexure from the number 

 of vibrations carried out by each in a certain time, he found that 

 wood possessed three axes of elasticity. These axes coincide with 

 the axes of calorific conduction established by the foregoing experi- 

 ments. The axis of greatest elasticity coincides with that of 

 highest conductive capacity, and the axis of least elasticity with 

 that of lowest conductive capacity. 



A few exact experiments made with a view to ascertain the 

 influence of molecular structure upon the cleavage of wood would 

 have formed an interesting addition to this communication ; for- 

 tunately, however, the mere sense of touch, to fingers accustomed 

 to seek for the cleavages of crystals, affords sufficient evidence 

 here. If a piece of wood be taken, on which the rings which 

 mark the growth of the tree plainly appear, and a penknife or a 

 chisel be laid across the rings, it will be found that the pressure 

 necessary to cleave the wood is less in this than in any other 



* Taylor's Scientific Memoirs, vol. i. p. 139. 



